green tea

I have always used the twinings green tea that comes in teabags and I was wondering (and I guess it is stupid question) if japanese green tea is very different from chinese or indian green tea... Also, is the flavor of tea made from tea leaves very different from that of tea made from teabags?

As someone who lives in Italy, you will certainly know a thing or two about coffee. How where it comes from, how it is stored, how it is processed, how it is blended, how it is ground and a whole number of other factors affect the taste of coffee.
It's the same with tea.
I'd go on to say that tea can vary as much as wine does for pretty much all the same reasons.

So, yes, Japanese tea can be very different from tea grown in China, India, Taiwan, Korea (and even the UK - there's a grower in the Southwest).
Japanese tea can be very different from Japanese tea also. There are so many variables involved it isn't always about which country it comes from.

I grew up having been taught that tea has no actual taste - if you block your nose it's tasteless as the flavour comes from its fragrance. This isn't necessarily true. Many of the better green teas have umami also, the 'taste' associated with kombu and parmigiano regianno and all the most flavoursome savoury foods.

Is tea from a bag different from loose leaf tea?
This really is tough to answer without going into pages.
It's not really the bag that has the biggest influence on the taste of tea - there really are so many other factors involved.
A teabag isn't the optimum way to brew tea, but it doesn't necessarily ruin tea either - not completely.
But it means that you are highly unlikely to ever find very good tea in a tea bag.

But just like with wine and coffee, sure there are the very best examples out there but you should have confidence in your own taste.
Whilst you might enjoy an espresso made with freshly ground arabica beans or appreciate a brew from beans that have passed through a civet's digestive tract, you might still be someone who actually likes to drink Nescafe powdered coffee dissolved in warm milk in the mornings.

(Ignoring matcha) Green tea is at its best when made from loose leaves.
However...
I personally like konacha - a tea dust that is usually sold in bags.

Sorry, esperanza, I'm finding it very hard to come to any kind of point with this answer.
Tea is such an enormous subject. I care very passionately about it and I already realise I can never know everything there is to know about tea, not even green tea.
I hope you will one day get to try properly made gyokuro, or try a variety of fine sencha, and get a chance to compare sencha from different estates with green teas grown in Taiwan or Korea.
And even the same tea will change very perceptibly depending on which temperature it is brewed at and how long for and after an infusion.
It could be that you will still quite like the taste of twinings green tea, or you could start disliking it after you get a taste for green tea more like the tea the Japanese themselves drink.

Whether the tea was grown in Japan or not or whether it comes in a bag simplifies the subject way too much - it really is a lot more complicated, and much, much more interesting than that.

The most knowledgeable (and most wonderfully enthusiastic) person I've ever met on the subject of tea has just had a book released in Japan. He is very generous with his knowledge and I've learned an awful lot from him. I have no doubt that the book is excellent. Unfortunately, I'll need to hope it gets translated to appreciate it myself.
But I just discovered that a mini-tutorial was filmed to promote it and this is in English.http://www.amazon.co.jp/%E7%A9%B6%E6%A5%B5%E3%81%AE%E7%B4%85%E8%8C%B6%E3...

To put into perspective, for me, Chinese green tea is more astringent (that dry feeling on your tongue) than Japanese green tea. Chinese g/t is delicious with honey, whereas Japanese g/t is best plain. I can't guarantee you'll like it; I had a friend who visited Tokyo with me and described it as "fish tank water". It tastes very green, indeed. Loose leaf has much more aroma than tea bag, and depending on the type of tea, you can get 2-3 extra brews.

thanks to everyone for the information! the subject now is a little bit more clear to me, even though I realize there is so much to know about it :-) the analogy with coffee made me understand how important green tea must be in the japanese culture... I bought a bag of bancha tea and I have just tried it: I really liked it, even though I am not sure if it is a regular kind of green tea or not...

I really liked it, even though I am not sure if it is a regular kind of green tea or not...

I really believe that this is the most important thing with tea, that you like it!

Bancha can be described as everyday green tea for a large percentage of Japanese people. It's not the tea used when visitors come round or given as a gift but it's still very nice to drink. Basically, the first Spring crop of tea leaves for sencha tea is called shincha (new tea), later in the year the same tea plants will be harvested again a few more times and it is during the later harvests that the leaves get used for the more economically priced bancha.
I think bancha is a perfect introduction to Japanese green teas, some people think the better shincha/sencha tastes are too much like the cooking water for cabbage or asparagus if they are unused to it. I believe you'll appreciate shincha better now you've developed a taste for bancha.

I tend to drink bancha which has been mixed with toasted rice - this kind of tea is called genmaicha.

The only tea I drink is Japanese 'cha;' but when I was a teenager, I bought all the different Twinings tins and particularly favored Gunpowder and Darjeeling. They were pleasant, but tannic; years intervened in which I was a coffee aficionado, then returned to tea but this time via Japan.

The website I most enjoy is Hibiki-an. Although it is commercial, it explains tea growing, harvesting, processing and tasting in more detail than one might believe possible! Sometimes I browse around on their site, just to dream of the tea harvests to come... and the ways they will store or offer other varieties, and on it goes... indeed difficult to come to a point on this subject.

This year, for the first time I ordered shincha, the first harvest of Sencha -- but since I am not a big Sencha lover, I chose my usual various orders of organic gyokuro, houjicha (a roasty flavor), and genmaicha -- and added shincha fukamushi -- meaning, the tea is steamed for _30 seconds more than the usual time_. It was ASTONISHING and utterly delicious due to, again, an indescribable quality which is why I seem to be maundering endlessly on about it!

Next year, or in the autumn of this year, they may offer 'aged' gyokuro, which I expect will be yet another sensation for which words will fail me. :-D

Do explore that site; it is marvelous. There is so much to know and appreciate in Japanese tea!